Screw chiller economizer system

ABSTRACT

Screw chillers have economizer systems that include a low pressure economizer and a high pressure economizer. According to certain embodiments, the low pressure economizer includes a flash tank, an expansion device and a flow control valve while the high pressure economizer includes a heat exchanger and an expansion device. The screw chillers also include a screw compressor that compresses refrigerant. The screw compressor includes a low pressure economizer port designed to receive lower pressure refrigerant from the flash tank within the low pressure economizer and a high pressure economizer port designed to receive higher pressure refrigerant from the heat exchanger within the higher pressure economizer. The screw compressor is designed to compress the refrigerant received from the evaporator, the refrigerant received through the low pressure economizer port, and the refrigerant received through the high pressure economizer port and to discharge the refrigerant through a common discharge.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from and the benefit of U.S. Provisional Application Ser. No. 61/408,752, entitled “SCREW CHILLER ECONOMIZER SYSTEM”, filed Nov. 1, 2010, which is hereby incorporated by reference

BACKGROUND

The invention relates generally to economizer systems for screw chillers.

Many applications exist for refrigeration systems including residential, commercial, and industrial applications. For example, a commercial refrigeration system may be used to cool an enclosed space such as a data center, laboratory, supermarket, or freezer. Generally, refrigeration systems may operate by circulating a fluid, such as refrigerant, through a closed loop between an evaporator where the fluid absorbs heat and a condenser where the fluid releases heat. The fluid flowing within the closed loop is generally formulated to undergo phase changes within the normal operating temperatures and pressures of the system so that considerable quantities of heat can be exchanged by virtue of the latent heat of vaporization of the fluid.

Some refrigeration and air conditioning systems rely on chillers to reduce the temperature of a process fluid, typically water. In such applications, the chilled water may be passed through downstream equipment, such as air handlers, to cool other fluids, such as air in a building. In typical chillers, the process fluid is cooled by an evaporator that absorbs heat from the process fluid by evaporating refrigerant. The refrigerant is then compressed by a compressor and transferred to a condenser. In the condenser, the refrigerant is cooled, typically by airflow, and recondenses into a liquid. Air cooled condensers typically comprise a condenser coil and a fan that induces airflow over the coil. In some conventional designs, an economizer is utilized in the chiller design to improve performance. For example, a flash tank economizer may be employed to at least partially evaporate a portion of the condensed refrigerant. The vapor phase refrigerant may be extracted from the flash tank and redirected to the compressor, while the liquid phase refrigerant from the flash tank is directed to the evaporator, closing the refrigeration loop. Although a flash tank economizer may improve performance, at extremely high ambient temperatures, additional performance enhancements may be desired.

SUMMARY

The present invention relates to a refrigeration system that includes a condenser configured to condense refrigerant, a first economizer system, a second economizer system, an expansion device configured to receive the refrigerant from the first economizer system or the second economizer system and to expand the refrigerant, an evaporator configured to evaporate the expanded refrigerant, and a screw compressor. The first economizer system includes a first expansion device configured to expand a first portion of the condensed refrigerant and a heat exchanger configured to subcool a second portion of the condensed refrigerant with the first portion. The second economizer system includes a second expansion device configured to expand the refrigerant and a flash tank configured to separate the refrigerant into vapor phase refrigerant and liquid phase refrigerant. The screw compressor includes a suction port configured to receive the evaporated refrigerant, a first economizer port configured to receive the first portion of the refrigerant, a second economizer port configured to receive the vapor phase refrigerant, and a discharge port. The screw compressor is configured to compress the evaporated refrigerant, the first portion of the refrigerant and the vapor phase refrigerant and to discharge the compressed refrigerant through the discharge port.

The present invention also relates to a refrigeration system that includes a condenser configured to condense refrigerant, a high pressure economizer system, and a lower pressure economizer system. The high pressure economizer system includes a high pressure expansion device configured to expand the condensed refrigerant and a high pressure flash tank configured to separate the expanded refrigerant into a high pressure vapor phase refrigerant and a high pressure liquid phase refrigerant. The low pressure economizer system includes a low pressure expansion device configured to expand the high pressure liquid refrigerant and a lower pressure flash tank configured to separate the expanded refrigerant into lower pressure vapor phase refrigerant and lower pressure liquid phase refrigerant. The refrigeration system further includes a third expansion device configured to receive and to expand the lower pressure liquid phase refrigerant, an evaporator configured to evaporate the expanded refrigerant from the third expansion device, and a screw compressor. The screw compressor includes a suction port configured to receive the evaporated refrigerant, a first economizer port configured to receive the high pressure vapor phase refrigerant, a second economizer port configured to receive the lower pressure vapor phase refrigerant, and a discharge port. The screw compressor is configured to compress the evaporated refrigerant, the high pressure vapor phase refrigerant, and the lower pressure vapor phase refrigerant and to discharge the compressed refrigerant through the discharge port.

The present invention further relates to a refrigeration system that includes a condenser configured to condense refrigerant, a high pressure economizer system, and a low pressure economizer system. The high pressure economizer system includes a first expansion device configured to expand a first portion of the condensed refrigerant and a heat exchanger configured to subcool a second portion of the condensed refrigerant with the first portion. The low pressure economizer system includes a low pressure expansion device configured to expand the subcooled second portion and a flash tank configured to separate the expanded second portion into vapor phase refrigerant and liquid phase refrigerant. The refrigeration system further includes a third expansion device configured to expand the liquid phase refrigerant, an evaporator configured to evaporate the expanded refrigerant, and a screw compressor. The screw compressor includes a suction port configured to receive the expanded refrigerant, a low pressure economizer port configured to receive the vapor phase refrigerant, a high pressure economizer port configured to receive the first portion of the refrigerant, and a discharge port. The screw compressor is configured to compress the refrigerant received through the suction port, the low pressure economizer port, and the high pressure economizer port and to discharge the compressed refrigerant through the discharge port.

DRAWINGS

FIG. 1 depicts an embodiment of a commercial heating ventilating, air conditioning and refrigeration (HVAC&R) system that employs a screw chiller with an economizer system in accordance with aspects of the present techniques.

FIG. 2 is a diagrammatical view of an embodiment of a screw chiller economizer system in accordance with aspects of the present techniques.

FIG. 3 is a diagrammatical view of another embodiment of a screw chiller economizer system in accordance with aspects of the present techniques.

FIG. 4 is a diagrammatical view of another embodiment of a screw chiller economizer system in accordance with aspects of the present techniques.

DETAILED DESCRIPTION

The present disclosure is directed to screw chillers with economizer systems that include a low pressure economizer and a high pressure economizer. According to certain embodiments, the low pressure economizer includes a flash tank, an expansion device and a flow control valve, while the high pressure economizer includes a heat exchanger and an expansion device. In certain embodiments, the high pressure economizer also may include a flow control valve. The screw chillers also include a screw compressor that compresses refrigerant received from an evaporator. The screw compressor includes a low pressure economizer port designed to receive lower pressure refrigerant from the low pressure economizer and a high pressure economizer port designed to receive higher pressure refrigerant from the high pressure economizer. The screw compressor is designed to compress the refrigerant received from the evaporator, the refrigerant received through the low pressure economizer port, and the refrigerant received through the high pressure economizer port and to discharge the compressed refrigerant through a common discharge port. The injection of the lower pressure refrigerant and the higher pressure refrigerant into the screw chiller through the economizer ports may be designed to reduce the mass flow rate of refrigerant through the lower pressure regions of the screw compressor, which in turn may reduce the load on the compressor, thereby providing increased system capacity. Further, the economizers may be designed to provide additional subcooling of the refrigerant, which in turn may increase the system efficiency. According to certain embodiments, the screw chillers disclosed herein may be particularly well suited to environments with high ambient temperatures.

FIG. 1 depicts an exemplary application for a refrigeration system. Such systems, in general, may be applied in a range of settings, both within the heating, ventilating, air conditioning, and refrigeration (HVAC&R) field and outside of that field. The refrigeration systems may provide cooling to data centers, electrical devices, freezers, coolers, or other environments through vapor-compression refrigeration, absorption refrigeration, or thermoelectric cooling. In presently contemplated applications, however, refrigeration systems may be used in residential, commercial, light industrial, industrial, and in any other application for heating or cooling a volume or enclosure, such as a residence, building, structure, and so forth. Moreover, the refrigeration systems may be used in industrial applications, where appropriate, for basic refrigeration and heating of various fluids.

FIG. 1 illustrates an exemplary application, in this case an HVAC&R system for building environmental management, that may employ heat exchangers. A building 10 is cooled by a system that includes a chiller 12 and a boiler 14. As shown, chiller 12 is disposed on the roof of building 10 and boiler 14 is located in the basement; however, the chiller and boiler may be located in other equipment rooms or areas next to the building. Chiller 12 is an air cooled or water cooled device that implements a refrigeration cycle to cool water. Chiller 12 is housed within a single structure that includes a refrigeration circuit, a free cooling system, and associated equipment such as pumps, valves, and piping. For example, chiller 12 may be single package rooftop unit. Boiler 14 is a closed vessel in which water is heated. The water from chiller 12 and boiler 14 is circulated through building 10 by water conduits 16. Water conduits 16 are routed to air handlers 18, located on individual floors and within sections of building 10.

Air handlers 18 are coupled to ductwork 20 that is adapted to distribute air between the air handlers and may receive air from an outside intake (not shown). Air handlers 18 include heat exchangers that circulate cold water from chiller 12 and hot water from boiler 14 to provide heated or cooled air. Fans, within air handlers 18, draw air through the heat exchangers and direct the conditioned air to environments within building 10, such as rooms, apartments, or offices, to maintain the environments at a designated temperature. A control device, shown here as including a thermostat 22, may be used to designate the temperature of the conditioned air. Control device 22 also may be used to control the flow of air through and from air handlers 18. Other devices may, of course, be included in the system, such as control valves that regulate the flow of water and pressure and/or temperature transducers or switches that sense the temperatures and pressures of the water, the air, and so forth. Moreover, control devices may include computer systems that are integrated with or separate from other building control or monitoring systems, and even systems that are remote from the building.

FIG. 2 schematically depicts an embodiment of chiller 12, which incorporates an economizer system. As noted above with respect to FIG. 1, chiller 12 is housed within a single structure and may be located outside of a building or environment, for example on a rooftop. Chiller 12 includes a cooling fluid loop 24 that circulates a cooling fluid, such as chilled water, an ethylene glycol-water solution, brine, or the like, to a cooling load, such as a building, piece of equipment, or environment. For example, cooling fluid loop 24 may circulate the cooling fluid to water conduits 16 shown in FIG. 1. In certain embodiments, the cooling fluid may circulate within the cooling fluid loop 24 to a cooling load, such as a research laboratory, computer room, office building, hospital, molding and extrusion plant, food processing plant, industrial facility, machine, or any other environments or devices in need of cooling.

Chiller 12 includes an evaporator 26 that transfers heat from the cooling fluid loop 24 to refrigerant flowing within a closed refrigerant loop 28. The refrigerant may be any fluid that absorbs and extracts heat. For example, the refrigerant may be a hydrofluorocarbon (HFC) based R-410A, R-407C, or R-134a, or it may be carbon dioxide (R-744) or ammonia (R-717). As the refrigerant flows through evaporator 26, the refrigerant absorbs heat from the cooling fluid flowing within evaporator 26 to cool the cooling fluid before the cooling fluid returns to the cooling load.

The heated refrigerant from evaporator 26 is circulated through refrigerant loop 28 to cool the refrigerant before returning the refrigerant to evaporator 26 where the refrigerant may again absorb heat from the cooling fluid. In particular, the heated refrigerant from evaporator 26 flows to a compressor 30 as a low pressure and temperature vapor. The refrigerant from evaporator 26 enters compressor 30 through a suction port 31. Compressor 30 reduces the volume available for the refrigerant vapor, consequently, increasing the pressure and temperature of the vapor refrigerant. According to certain embodiments, compressor 30 may be a screw compressor, such as a tri-rotor screw compressor. Compressor 30 is driven by a motor 32 that receives power from a variable speed drive (VSD) or from a direct AC or DC power source. According to certain embodiments, motor 32 receives fixed line voltage and frequency from an AC power source. Further, in certain applications, motor 32 may be driven by a variable voltage or frequency drive. The motor may be a switched reluctance (SR) motor, an induction motor, an electronically commutated permanent magnet motor (ECM), or any other suitable motor type.

Compressor 30 includes two economizer ports 34 and 36 that also direct refrigerant into compressor 30. Economizer port 34 receives lower pressure refrigerant from a low pressure economizer system 38, and economizer port 36 receives higher pressure refrigerant from a high pressure economizer system 40. Each of the economizer ports 34 and 36 direct refrigerant into a different pressure area within compressor 30. For example, economizer port 36 may direct refrigerant into a higher pressure area than economizer port 34, and economizer port 34 may direct refrigerant into a higher pressure area than suction inlet 31. Further, the pressure of the refrigerant entering through each economizer port 34 and 36 is higher than the pressure of the refrigerant within the compression area coupled to each economizer port 34 or 36. Accordingly, relatively higher pressure refrigerant enters compressor 30 through economizer ports 34 and 36 to mix with existing refrigerant of a relatively lower pressure.

In certain embodiments, the pressure difference may be designed to draw refrigerant into compressor 30 through economizer ports 34 and 36. For example, the refrigerant entering compressor 30 through economizer port 34 may be at a pressure higher than the pressure of the refrigerant within the compression chamber coupled to economizer port 34. Accordingly, the higher pressure refrigerant entering through economizer port 34 may be drawn into the compressor 30, where the refrigerant may mix with the existing refrigerant and expand within the constant volume of compressor 30, thereby further increasing the pressure within compressor 30. In a similar example, the refrigerant entering compressor 30 through economizer port 36 may be at a higher pressure than the pressure of the refrigerant within the compression area coupled to economizer port 36. The pressure difference may draw the higher pressure refrigerant into compressor 30, where the higher pressure refrigerant may mix with the existing refrigerant and expand to further increase the pressure within compressor 30. As the refrigerant from suction inlet 31 and from economizer ports 34 and 36 flows through compressor 30, the refrigerant is compressed. The compressed refrigerant from suction inlet 31 and from economizer ports 34 and 36 is discharged through a common discharge port 37 that directs the compressed refrigerant to a condenser 42.

The compressed refrigerant enters condenser 42 as a high pressure and temperature vapor. According to certain embodiments, condenser 42 may be a fin and tube heat exchanger, brazed aluminum multichannel heat exchanger, or other suitable heat exchanger. A fan 44, which is driven by a motor 46, draws air across condenser 42 to cool the refrigerant flowing within condenser 42. According to certain embodiments, the refrigerant vapor may condense to a liquid as the refrigerant flow through condenser 42. The liquid refrigerant then flows through economizer systems 38 and 40, where the refrigerant may be subcooled.

After flowing through economizer systems 38 and 40, the liquid refrigerant flows through an expansion device 48 where the refrigerant expands to become a low pressure and temperature liquid. Typically, expansion device 48 will be a thermal expansion valve (TXV); however, according to other exemplary embodiments, the expansion device may be an electronic expansion valve, a fixed orifice, or the like. According to certain embodiments, after the refrigerant exits the expansion device, some vapor refrigerant may be present in addition to the liquid refrigerant. From expansion device 48, the refrigerant flows through evaporator 26 where the refrigerant may again absorb heat from the cooling fluid flowing within cooling fluid loop 24.

Economizer systems 38 and 40 may be designed to provide additional subcooling of the refrigerant entering evaporator 26, which, in turn may increase the efficiency of chiller 12. Economizer systems 38 and 40 also may be designed to direct a portion of the refrigerant back to compressor 30, bypassing evaporator 26, which may reduce the mass flow rate of refrigerant in the lower pressure regions of compressor 30, thereby reducing the load on compressor 30.

High pressure economizer system 40 includes a heat exchanger 50, an optional flow control valve 52, and an expansion device 54. According to certain embodiments, heat exchanger 50 may be a brazed plate heat exchanger; however, in other embodiments, any suitable type of heat exchanger may be employed. High pressure economizer system 40 receives the condensed refrigerant from condenser 42 and separates the refrigerant into a first portion that directly enters heat exchanger 50 and a second portion that flows through optional flow control valve 52 and expansion device 54 prior to entering heat exchanger 50. As the second portion of the refrigerant flows though expansion device 54, the refrigerant is expanded, thereby reducing the pressure and temperature of the refrigerant. According to certain embodiments, expansion device 54 may be a thermal expansion valve (TXV), an electronic expansion valve, or a fixed orifice, among others. In certain embodiments, an optional flow control valve 52 may be disposed upstream of expansion device 54 to control the flow of refrigerant within high pressure economizer system 40. Flow control valve 52 may be a solenoid valve, ball valve, gate valve, rotor valve, or the like, controlled by electromechanical actuators, pneumatic actuators, hydraulic actuators, or other suitable controls. In other embodiments, for example, where expansion device 54 is an electronic expansion valve, flow control valve 52 may be omitted and expansion device 54 may be used to control the flow of refrigerant through economizer system 40.

From expansion device 54, the second portion of refrigerant flows through heat exchanger 50 where the lower pressure second portion of refrigerant absorbs heat from the first portion of refrigerant that directly enters heat exchanger 50 from condenser 42. As the second portion of the refrigerant is heated within heat exchanger 50, the second portion of refrigerant may evaporate to produce vapor refrigerant that is directed to compressor 30 through economizer port 36. As noted above, the refrigerant entering compressor 30 through economizer port 36 may have a higher pressure than the existing refrigerant within compressor 30. The higher pressure may draw the refrigerant into compressor 30 and also may further increase the pressure of the refrigerant within compressor 30.

As the first portion of refrigerant flows through heat exchanger 50, the first portion of refrigerant transfer heats to the second portion of refrigerant flowing through heat exchanger 50. Accordingly, the first portion of refrigerant flowing through heat exchanger 50 may be subcooled by the second portion of refrigerant that has passed through expansion device 54. According to certain embodiments, heat exchanger 50 approximates a counter-flow configuration to maximize heat transfer. The subcooled refrigerant exiting heat exchanger 50 is then directed to the lower pressure economizer system 38, where the refrigerant may undergo further subcooling. In other embodiments, the second portion of refrigerant that is directed through expansion device 54 may be drawn from a point downstream of heat exchanger 50. For example, the first portion of refrigerant may flow through heat exchanger 50. The second portion of refrigerant may then be drawn from the first portion of refrigerant that has exited heat exchanger 50. The second portion of refrigerant may then be directed through expansion device 54 and heat exchanger 50. According to certain embodiments, the additional subcooling may improve operation of the expansion device 54.

Low pressure economizer system 38 includes a flash tank 56, an expansion device 58, and a flow control valve 60. Within low pressure economizer system 38, the refrigerant flows through an expansion device 58 where the refrigerant is expanded to reduce the pressure and temperature of the refrigerant. According to certain embodiments, expansion device 58 may be a thermal expansion valve, electronic expansion valve, fixed orifice, or the like. Further, in certain embodiments, a controller 62 may operate expansion device 58 to limit the level of liquid refrigerant in flash tank 56. The refrigerant from expansion device 58 then enters flash tank 56 where the vapor and liquid phases within the refrigerant may be separated. The vapor phase refrigerant exits flash tank 56 through an upper portion of flash tank 56 and flows through a flow control valve 60, which may control the flow of refrigerant through low pressure economizer system 38. Flow control valve 60 may be a solenoid valve, ball valve, gate valve, rotor valve, or the like, controlled by electromechanical actuators, pneumatic actuators, hydraulic actuators, or other suitable controls. From flow control valve 60, the vapor phase refrigerant is directed to compressor 30 through economizer port 34. As noted above, the refrigerant entering compressor 30 through economizer port 34 may have a higher pressure than the pressure of the existing refrigerant in the compression area coupled to economizer port 34. The relatively higher pressure may draw the refrigerant into compressor 30 and also may further increase the pressure of the refrigerant within compressor 30.

Within flash tank 56, the liquid phase refrigerant may separate from the vapor phase refrigerant and collect within a lower portion of flash tank 56. From flash tank 56, the liquid phase refrigerant is directed through expansion device 48 and returned to evaporator 26, where the liquid phase refrigerant may absorb heat from the cooling fluid flowing through cooling fluid loop 24.

Operation of economizer systems 38 and 40 may be controlled by one or more controllers 62, which may receive information from input devices 64 and 66. For example, input device 64 may be a temperature sensor that measures the ambient air temperature, and input device 66 may be a temperature sensor that measures a temperature of the fluid within cooling fluid loop 24. However, in other embodiments, control of economizer systems 38 and 40 may be based on other input devices, such as a level sensor, pressure sensors, and/or pressure and temperature transducers in communication with controller 62.

Controller 62 may adjust operation of the economizer systems 38 and 40 based on input from input devices 62 and 64. Controller 62 is coupled to flow control valve 52 within high pressure economizer system 40 and to flow control valve 60 within low pressure economizer system 38. Controller 62 may open and close flow control valves 52 and 60 to enable and disable operation of one or both economizer system 38 and 40 based on control signals received from input devices 64 and 66. For example, controller 62 may enable both economizer systems 38 and 40 when ambient temperatures are high and/or when the temperature within the cooling fluid loop 24 is high. If the ambient temperature and/or the cooling fluid temperature decrease, controller 62 may close one or both valves 52 and 60 to bypass one or both of the economizer system 38 and 40. For example, when flow control valve 52 is closed, all of the refrigerant from condenser 42 may flow directly to heat exchanger 50 and no refrigerant may enter compressor 30 through economizer port 36. In another example, when flow control valve 60 is closed, all of the refrigerant exiting heat exchanger 50 may flow through flash tank 56 to expansion device 48, and no refrigerant may enter compressor 30 through economizer port 34. In certain embodiments, controller 62 may adjust valves 52 and 60 between open and closed positions. Further, in certain embodiments, controller 62 may adjust the amount that valves 52 and 60 are open to regulate the amount of flow to compressor 30 through economizer ports 34 and 36.

Controller 62 may execute hardware or software control algorithms to regulate operation of chiller 12. According to exemplary embodiments, controller 62 may include an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board. Controller 62 also includes, or is associated with, input/output circuitry for receiving sensed signals from input devices 64 and 66, and interface circuitry for outputting control signals for valves 52 and 60 and motors 32 and 46. Other devices may, of course, be included in the system, such as additional pressure and/or temperature transducers or switches that sense temperatures and pressures of the refrigerant, the heat exchangers, the compressor, the flash tank, the inlet and outlet air, and so forth. Further, other values and/or set points based on a variety of factors, such as system capacity, cooling load, and the like may be used to determine when to operate economizer systems 38 and 40. Moreover, according to certain embodiments, controller 62 also may be coupled to motor 46 of condenser 42 and to motor 32 of compressor 30. In these embodiments, controller 62 also may govern operation of condenser 42 and compressor 30 based on input received from input devices 62 and 64.

FIG. 3 depicts another embodiment of chiller 12 that incorporates high pressure economizer system 40 and low pressure economizer system 38. The embodiment shown in FIG. 3 is generally similar to the embodiment described above with respect to FIG. 2. However, rather than including a heat exchanger as shown in FIG. 2, the high pressure economizer system 40 shown in FIG. 3 includes a flash tank 68, an expansion device 70, and a flow control valve 72. According to certain embodiments, expansion device 70 may be a thermal expansion valve, electronic expansion valve, fixed orifice, or the like.

Condensed refrigerant from condenser 42 enters high pressure economizer system 40 and flows through expansion device 70 where the refrigerant is expanded to reduce the pressure and temperature of the refrigerant. The refrigerant then enters flash tank 68, where the liquid and vapor phases separate. The vapor phase refrigerant exits flash tank 68 and flows through control valve 72 to enter compressor 30 through economizer port 36. The liquid phase refrigerant collects within a lower portion of flash tank 68 and is directed to low pressure economizer system 38.

FIG. 4 depicts an embodiment of chiller 12 that includes a heat exchanger within high pressure economizer system 40 and a flash tank within low pressure economizer system 38. The embodiment of chiller 12 shown in FIG. 4 is generally similar to the embodiment of chiller 12 shown in FIG. 3; however, the positions of the flash tank and the heat exchanger are interchanged.

As shown in FIG. 4, condensed refrigerant from condenser 42 enters high pressure economizer system 40 and flows through expansion device 70, where the refrigerant is expanded. The refrigerant then enters a flash tank 68 where the vapor and liquid phase refrigerant separates. The vapor phase refrigerant flows through a flow control valve 72 to enter compressor 30 through economizer port 36, while the liquid phase refrigerant flows from flash tank 68 to low pressure economizer system 38.

Within low pressure economizer system 38, the liquid phase refrigerant from flash tank 68 is separated into a first portion that flows directly to a heat exchanger 74 and a second portion that flows through an optional flow control valve 76 and an expansion device 78 prior to entering heat exchanger 74. According to certain embodiments, heat exchanger 74 may be a brazed plate heat exchanger; however, in other embodiments, any suitable type of heat exchanger may be employed. Expansion device 78 may be a thermal expansion valve (TXV), an electronic expansion valve, or a fixed orifice, among others. Flow control valve 76 may be a solenoid valve, ball valve, gate valve, rotor valve, or the like, controlled by electromechanical actuators, pneumatic actuators, hydraulic actuators, or other suitable controls. In other embodiments, for example, where expansion device 78 is an electronic expansion valve, flow control valve 76 may be omitted and expansion device 78 may be used to control the flow of refrigerant through economizer system 38.

As the second portion of refrigerant is expanded within expansion device 78, the pressure and temperature of the refrigerant may be reduced. Accordingly, within heat exchanger 74, the expanded second portion of refrigerant may absorb heat from the first portion of refrigerant that flows directly from flash tank 68 to heat exchanger 74. As the second portion of the refrigerant is heated within heat exchanger 50, the second portion of refrigerant may evaporate to produce vapor refrigerant that is directed to compressor 30 through economizer port 34. The first portion of refrigerant may be subcooled within heat exchanger 74 as the first portion of refrigerant transfers heat to the second portion of refrigerant. The subcooled refrigerant may then be directed through expansion device 48 to evaporator 26.

While only certain features and embodiments of the invention have been illustrated and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (i.e., those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation. 

1. A refrigeration system, comprising: a condenser configured to condense refrigerant; a first economizer system comprising a first expansion device configured to expand a first portion of the condensed refrigerant and a heat exchanger configured to subcool a second portion of the condensed refrigerant with the first portion; a second economizer system comprising a second expansion device configured to expand the refrigerant and a flash tank configured to separate the refrigerant into vapor phase refrigerant and liquid phase refrigerant; an expansion device configured to receive the refrigerant from the first economizer system or the second economizer system and to expand the refrigerant; an evaporator configured to evaporate the expanded refrigerant; and a screw compressor comprising a suction port configured to receive the evaporated refrigerant, a first economizer port configured to receive the first portion of the refrigerant, a second economizer port configured to receive the vapor phase refrigerant, and a discharge port, wherein the screw compressor is configured to compress the evaporated refrigerant, the first portion of the refrigerant, and the vapor phase refrigerant and to discharge the compressed refrigerant through the discharge port.
 2. The refrigeration system of claim 1, wherein the first economizer system is disposed upstream from the second economizer system and downstream of the condenser.
 3. The refrigeration system of claim 1, wherein the second economizer system is disposed upstream from the first economizer system and downstream of the condenser.
 4. The refrigeration system of claim 1, wherein the first economizer port comprises a lower pressure port relative to the second economizer port.
 5. The refrigeration system of claim 1, wherein the first economizer port comprises a higher pressure port relative to the second economizer port.
 6. The refrigeration system of claim 1, wherein the condenser comprises an air cooled condenser.
 7. The refrigeration system of claim 1, wherein the first economizer system comprises a flow control valve configured to control flow of the first portion of the refrigerant to the first economizer port.
 8. The refrigeration system of claim 1, wherein the second economizer system comprises a flow control valve configured to control flow of the vapor phase refrigerant to the second economizer port.
 9. A refrigeration system, comprising: a condenser configured to condense refrigerant; a high pressure economizer system comprising a high pressure expansion device configured to expand the condensed refrigerant and a high pressure flash tank configured to separate the expanded refrigerant into a high pressure vapor phase refrigerant and a high pressure liquid phase refrigerant; a low pressure economizer system comprising a lower pressure expansion device configured to expand the high pressure liquid refrigerant and a lower pressure flash tank configured to separate the expanded refrigerant into lower pressure vapor phase refrigerant and lower pressure liquid phase refrigerant; a third expansion device configured to receive and to expand the lower pressure liquid phase refrigerant; an evaporator configured to evaporate the expanded refrigerant from the third expansion device; and a screw compressor comprising a suction port configured to receive the evaporated refrigerant, a first economizer port configured to receive the high pressure vapor phase refrigerant, a second economizer port configured to receive the lower pressure vapor phase refrigerant, and a discharge port, wherein the screw compressor is configured to compress the evaporated refrigerant, the high pressure vapor phase refrigerant, and the lower pressure vapor phase refrigerant and to discharge the compressed refrigerant through the discharge port.
 10. The refrigeration system of claim 9, wherein the screw compressor is configured to draw the vapor phase refrigerant into the screw compressor based on a pressure difference between a pressure of the vapor phase refrigerant and a pressure in a compression area of the screw compressor coupled to the low pressure economizer port.
 11. The refrigeration system of claim 9, wherein the high pressure economizer system comprises a first flow control valve disposed downstream of the high pressure flash tank, wherein the first flow control valve is configured to control flow of the high pressure vapor phase refrigerant to the first economizer port, wherein the low pressure economizer system comprises a second flow control valve disposed downstream of the lower pressure flash tank, and wherein the second flow control valve is configured to control flow of the lower pressure vapor phase refrigerant to the second economizer port.
 12. The refrigeration system of claim 11, comprising a controller operably coupled to the first and second flow control valves and configured to govern operation of the first control valve to enable and disable operation of the high pressure economizer system, and to govern operation of the second control valve to enable and disable operation of the lower pressure economizer system.
 13. The refrigeration system of claim 9, comprising a controller configured to operate the high pressure expansion device to limit a level of liquid refrigerant in the high pressure flash tank, and to operate the low pressure expansion device to limit a level of liquid refrigerant in the low pressure flash tank.
 14. The refrigeration system of claim 9, wherein the condenser comprises an air cooled multichannel heat exchanger.
 15. A refrigeration system, comprising: a condenser configured to condense refrigerant; a high pressure economizer system comprising a first expansion device configured to expand a first portion of the condensed refrigerant and a heat exchanger configured to subcool a second portion of the condensed refrigerant with the first portion; a low pressure economizer system comprising a low pressure expansion device configured to expand the subcooled second portion and a flash tank configured to separate the expanded second portion into vapor phase refrigerant and liquid phase refrigerant; a third expansion device configured to expand the liquid phase refrigerant; an evaporator configured to evaporate the expanded refrigerant; and a screw compressor comprising a suction port configured to receive the expanded refrigerant, a low pressure economizer port configured to receive the vapor phase refrigerant, a high pressure economizer port configured to receive the first portion of the refrigerant, and a discharge port, wherein the screw compressor is configured to compress the refrigerant received through the suction port, the low pressure economizer port, and the high pressure economizer port and to discharge the compressed refrigerant through the discharge port.
 16. The refrigeration system of claim 15, wherein the condensed refrigerant is separated into the first portion of the condensed refrigerant and the second portion of the condensed refrigerant upstream of the heat exchanger.
 17. The refrigeration system of claim 15, wherein the first and second portions of the condensed refrigerant both flow through the heat exchanger.
 18. The refrigeration system of claim 15, wherein the screw compressor is configured to mix the first portion of the refrigerant received through the high pressure economizer port with existing refrigerant in the screw compressor, and wherein the screw compressor is configured to mix the vapor phase refrigerant received through the low pressure economizer port with existing refrigerant in the screw compressor.
 19. The refrigeration system of claim 15, wherein the high pressure economizer system comprises a first flow control valve disposed upstream of the first expansion device, wherein the flow control valve is configured to control flow of the first portion of the condensed refrigerant to the high pressure economizer port, wherein the low pressure economizer system comprises a second flow control valve disposed downstream of the flash tank, and wherein the second flow control valve is configured to control flow of the vapor phase refrigerant to the low pressure economizer port
 20. The refrigeration system of claim 19, comprising a controller operably coupled to the first and second flow control valves and configured to govern operation of the first and second flow control valves based on an ambient air temperature, or a temperature of the refrigerant, or both. 